A patient lacks small-intestinal lactase activity. Predict what happens to ingested lactose and which symptoms are most likely to develop shortly after milk consumption.

Lactose is absorbed intact across the small intestine via facilitated diffusion and causes systemic hypoglycemia because it bypasses digestion; therefore, the main symptom is fainting but no GI upset.

Lactase deficiency causes immediate stomach acid neutralization leading to heartburn without gas or diarrhea because the lactose directly neutralizes gastric acid.

Undigested lactose reaches the colon where bacteria ferment it, producing gas, osmotically active metabolites, and short-chain fatty acids, leading to bloating, cramping, and diarrhea.

Lactose spontaneously polymerizes into glycogen in the stomach preventing absorption, which causes constipation and decreased stool frequency as the primary symptom.